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EDX is the primary adjunctive diagnostic modality in the evaluation of the patient with a suspected lower extremity mononeuropathy The goals are to identify the existence of nerve injury if possible, localize the injury both to and within the course of an individual nerve, de ne the pathophysiology if possible, and, by doing so, estimate prognosis and aid in management decisions There are no reliable, routinely performed nerve conduction study techniques that assess the function of ilioinguinal, iliohypogastric, genitofemoral, pudendal, or posterior cutaneous nerve of the thigh In the appropriate clinical context, needle electromyographic evidence of denervation con ned to abdominal muscles supports but does not prove the existence of an ilioinguinal or iliohypogastric mononeuropathy Although it is theoretically possible to do an electromyographic evaluation of the cremaster muscle to support the diagnosis of a genitofemoral mononeuropathy, this is rarely if ever performed Needle electromyography of the external anal sphincter can be done but may be dif cult to interpret It res tonically, thereby making electrical silence dif cult to achieve and brillation potentials dif cult to detect The small motor unit action potentials that populate this muscle may also confound the detection of brillation potentials Sensory nerve conduction studies of the lateral cutaneous nerve of the thigh have been described and are

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fairly reproducible Although potentially technically dif cult in overweight individuals, identi cation of a SNAP of lateral cutaneous nerve of the thigh on the asymptomatic side with an absent or reduced amplitude of the analogous response on the symptomatic side provides strong diagnostic support in the appropriate clinical context EDX support for a femoral mononeuropathy is based primarily on the ndings of acute and or chronic changes of denervation con ned to femoral innervated muscles Femoral motor conduction studies can be performed but may be more uncomfortable than many other conduction studies if performed percutaneously in overweight individuals Additionally, there may be considerable side-to-side variation in CMAP amplitudes within a normal population, rendering convincing side-to-side amplitude comparisons dif cult One potential bene t of femoral motor conductions is in the patient with severe quadriceps weakness secondary to a demyelinating conduction block in the proximal femoral nerve If such a patient were studied more than 7 10 days after the onset of weakness, a CMAP amplitude from the symptomatic side, which approximated that of the asymptomatic side, associated with involuntary knee extension far greater than the patient can voluntarily generate, would generate one of two conclusions These would be either a conduction block injury with its associated optimistic prognosis or, alternatively, malingering Other features including the presence or absence of a patellar re ex or neurogenic recruitment pattern on needle examination would aid in distinguishing between the two possibilities The other EDX parameter of interest in a suspected femoral neuropathy would be the saphenous sensory response Its value is also slightly diminished by its inability to be detected in a certain percentage of normal individuals, particularly those that are older and endomorphic A unilateral abnormality in a symptomatic leg, however, would once again strongly support the existence of an axon loss femoral mononeuropathy, assuming that the clinical and needle electromyographic patterns were also compatible with this conclusion A unilateral abnormality of the saphenous SNAP would also be compatible with a lumbar plexopathy or lumbosacral trunk lesion Conversely, a normal saphenous SNAP would be consistent with a more proximally located, predominantly demyelinating neuropathy There are no motor or sensory conduction studies that are available for the assessment of obturator neuropathies EDX ndings are limited to denervation in obturator-innervated muscles with axon loss injury, or reduced motor unit recruitment in the unlikely possibility of a demyelinating pathophysiology with conduction block Sciatic neuropathies commonly occur at the level of the proximal thigh or buttock and are often incomplete It has been repeatedly stressed that the bers destined

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to become the peroneal nerve are anatomically separated even at this level and more vulnerable to most injuries than their tibial nerve counterparts140,141 Theoretically, with axon loss sciatic nerve lesions, both peroneal and tibial CMAPs and super cial peroneal and sural SNAPs should be reduced or absent on the affected side as well as the mixed nerve plantar responses if tested In reality, not all components are always affected or equally affected Along the same lines, all muscles below the knee as well as all four hamstrings should be affected with sparing of muscles innervated by the femoral, obturator, superior, and inferior gluteal nerves Again, denervative changes may be more patchy than anticipated As previously mentioned, the most precise localization of focal nerve lesions occurs when these are demyelinating in nature In the lower extremity, a common peroneal neuropathy occurring at the bular head is the only demyelinating mononeuropathy that will be demonstrable with any frequency Approximately 45% of common peroneal neuropathies will have at least a component of demyelinating conduction block62 It may be necessary to record from the tibialis anterior rather than the traditional EDB in order to demonstrate this With an axon loss common peroneal neuropathy occurring at the bular head, reduced CMAP amplitudes recording from both the EDB and the tibialis anterior muscles are anticipated The super cial peroneal SNAP will be reduced or absent in axon loss lesions, whereas it will be relatively spared in predominantly demyelinating lesions Changes of active and/or chronic partial denervation and reinnervation of muscles innervated by both the deep peroneal and the super cial peroneal nerves below the knee will be evident in axon loss lesions It may be recalled that many partial injuries to the sciatic nerve preferentially injure the peroneal division In this situation, both the clinical examination and the nerve conduction study results may appear similar to that expected in an axon loss common peroneal neuropathy Denervation in the short head of the biceps, the only peroneal-innervated muscle above the knee, provides a major clue by which to distinguish between these two entities Predominantly demyelinating common peroneal neuropathies occur fairly commonly CMAP amplitudes recording from either the EDB or the tibialis anterior are typically normal or mildly reduced if any component of axon loss is present It should be emphasized that when conduction block occurs, it tends to do so slightly distal to the prominence of the bular head To adequately identify it, the electromyographer may have to make an effort to move the stimulator to a slightly more distal location than typically used, in order to demonstrate the abrupt increase in CMAP amplitude that can be elicited below the lesion Super cial peroneal SNAPs are similarly spared in predominantly demyelinating